Light emitting diodes (LEDs) are widely used in portable devices (e.g., cell phones) for various applications. For example, white LEDs (WLEDs) are often used for backlighting liquid crystal display (LCD) screens and dimming keypads in portable devices. Under many circumstances, it is important to have uniform color/luminous intensity across an LCD screen. Because color and luminous intensity of an LED depend on an average current flowing through the LED, all LEDs used for backlighting the LCD screen usually need to have similar average currents to keep color/luminous uniformity.
There are many approaches for current matching of LEDs. For example, conventionally, multiple LED strings may be used in parallel, where each LED string is connected with a current sink. Current matching is achieved through trimming the current sinks. As another example, a power converter, e.g., a boost converter, can be used to drive multiple LED strings for current matching. A pulse-frequency-modulation (PFM) topology may be implemented in the power converter.
The PFM converter can operate with different switching frequencies depending on load conditions. For example, the switching frequency of the PFM converter is higher for a heavy load than that for a light load. One disadvantage of the PFM converter is that audible noise may be generated when the switching frequency is very low under a light-load/no-load condition. A pulse-width-modulation (PWM) topology, which often uses a fixed frequency, may be implemented in the power converter to reduce audible noise. However, it too has a number of disadvantages. Efficiency of a PWM converter, for example, is often much lower than that of the PFM converter. Also, the PWM converter usually needs bulky external components which are not suitable for portable devices. In addition, when a power converter is used to drive multiple LED strings, audible noise may be generated from voltage ripples when the LED strings need different output voltages and have different duty cycles.
An improved method to drive LEDs using a power converter (e.g., a PFM power converter) with reduced audible noise is highly desirable.